We utilized high throughput screening technologies to help identify compounds and extracts that can specifically interact with or modulate the function of selected biochemical targets or processes. Bioassay-guided chemical fractionation of natural products extracts is employed to isolate and purify the individual bioactive compounds. Identification and structural characterization of these compounds provides new structural classes or molecular scaffolds for the development of potential drug leads or biological probes that can interact with the desired molecular target. In addition to extensive NMR and mass spectroscopic analyses, our efforts include rigorous evaluation of a new compound's potency, molecular target specificity, and mode of action. A new class of marine alkaloids named the eudistidines was identified and these metabolites were shown to disrupt interactions between the transcriptional co-activator p300 and HIF-1 alpha. These compounds have an unprecedented fused tetracyclic core skeleton, comprised of two pyrimidine rings fused with an imidazole ring that also contains embedded guanidine and amidine functionalities. The novel molecular architecture of eudistidine C was initially assigned from a comprehensive analysis of spectroscopic data (primarily NMR and MS). Elucidation of the structure presented some very significant challenges due to a lack of protonated carbon and nitrogen atoms in the core of the molecule. Application of sophisticated NMR experiments and pains-taking data analysis finally allowed assignment of the novel structures. A synthetic effort to verify the structure and provide additional material for biological evaluates was accomplished in conjunction with staff in the Chemical Biology Laboratory. The novel heterocyclic architecture of eudistidine C was confirmed by a one-step synthesis that involved reaction of eudistidine A with a methoxyphenyl-aminoimidazole reagent to generate a synthetic product that was identical in all respects with the natural product. The eudistidine C scaffold, which can now be synthesized in a straightforward and scalable manner, may provide potential therapeutic lead compounds or molecular probes to study p300/HIF-1 alpha interactions and the role these proteins play in tumor response to low oxygen conditions. An EIR was filed to cover the discovery and synthesis of the eudistidines and a high impact publication that describes their isolation, structural elucidation, and synthesis has been prepared. A series of semisynthetic plant triterpene derivatives were obtained from Professor Jorge Salvador at the University of Coimbra, Portugal and tested in all of the MTL and DTP assays. One of these compounds showed selective cytotoxic activity against melanoma and colon cancer cell lines and Cell Miner bioinformatic analysis of the NCI 60-cell data showed a strong correlation with clinically used B-Raf inhibitors. All of the sensitive cell lines have V600E mutated B-Raf (constitutive active B-Raf). The clinical agents inhibit the kinase activity of B-Raf. The compound we investigated did not inhibit the kinase activity of B-Raf or C-Raf, but it did result in significant reductions in cellular levels of B-Raf and C-Raf. The focus of this project is to exploit the vast spectrum of chemical diversity within the NPR for potential anticancer and anti-HIV applications. It relies on close integration with the MTP Assay Development and Screening Section, Chemical Diversity Development Section, and the Protein Chemistry and Molecular Biology Section for extract screening, data analysis, bioassay support, and functional analysis of isolated compounds. Our CCR collaborators who study aspects of cancer biology, genetics, and immunology provide expertise for target selection and subsequent compound evaluation. We have assembled a broad consortium of intramural and extramural partners with expertise in organic synthesis, chemical biology, molecular pharmacology, computational sciences, and spectroscopic analysis to help characterize and advance our natural product discoveries. With 25 years of experience studying the chemistry of extracts from the NPR, a proven capability to develop and apply targeted screening assays, and access to CCR investigators with broad expertise in cancer and HIV biology, the Natural Products Chemistry Section is uniquely positioned within the NCI to combine molecular target-based discovery with natural products chemistry. Natural products are a source of structural complexity and biological activity that can provide insight on the function of new targets, pathways, or cellular processes. They play an important role in dissecting and understanding the intricacies of cancer development and progression, so continued natural products discovery efforts can complement the goals of the CCR and NCI.